Internal combustion engines are lubricated by circulating lubricating oil (or crankcase lubricant) from an oil sump generally situated below the crankshaft of the engine. To reduce the energy and fuel requirements of the engine, there is a need for 110 crankcase lubricants that reduce the overall friction of the engine.
U.S. Pat. No. 6,423,671 ('671) relates to lubricating compositions with improved frictional characteristics which translates into improved fuel economy when the compositions are used in internal combustion engines. In particular, '671 relates to lubricant compositions containing organo-molybdenum compounds together with zinc salts, metal-containing detergents and ashless friction modifiers (referred to as surfactants). '671 states that molybdenum compounds can improve frictional characteristics but that their effect is not fully realised in the above particular compositions because of preferred absorption on moving surfaces of the non-molybdenum polar components. This competition for absorption of polar components results, for example, in a tendency for detergents to be absorbed more readily then molybdenum compounds.
'671 meets the above problem by using dispersants to form a first semi-package with the above-mentioned non-molybdenum polar components, the semi-package being made by mixing and heating the components, for example at about 90° C. for about 1-3 hours. The molybdenum component is provided in a second semi-package, and the first and second semi-packages added to an oil of lubricating viscosity.
A problem with the approach described in '671 in that it requires additional processing steps, particularly the preparation of the first semi-package. The present invention meets the problem of competition for absorption in a different way, namely by employing a detergent system of low metal ratio and a lubricating oil composition of low total base number (TBN). Surprisingly, considerably better coefficient of friction results are achieved, as evidenced by the data in this specification.